Bugfix for NullPointerException when xmp:ModifyDate isn't set in the FO metadata.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@415981 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/fop/pdf/PDFMetadata.java

@@ -234,7 +234,9 @@ public class PDFMetadata extends PDFStream {
         //than Info/CreationDate
         info.setCreationDate(d);
         d = xmpBasic.getModifyDate();
-        xmpBasic.setModifyDate(d);
-        info.setModDate(d);
+        if (d != null) { //ModifyDate is only required for PDF/X
+            xmpBasic.setModifyDate(d);
+            info.setModDate(d);
+        }
     }
 }